Conventional cemented carbide alloy machine tools and abrasion resistant component parts are produced by press molding a predominantly tungsten carbide powder composition and sintering the resulting molded mass by powder metallurgy techniques. However, because of the limited availability of tungsten ore, sintered alloys made therefrom are relatively expensive. To reduce the extent of tungsten carbide usage and in order to modify the properties of these alloys, titanium compounds have been alloyed with tungsten carbide. Moreover, titanium compounds, such as titanium nitride, carbide and carbonitride appear to have properties, such as hardness, melting point, and density, equivalent or comparable to the properties of tungsten carbide which would make them useful and desirable in lieu of tungsten carbide as cutting tool materials. However, titanium compound alloys have not met with the success and acceptance that might have been expected primarily because of inconsistency in the quality of the starting material titanium compound powders.
Wear resistant coatings of titanium carbide and nitride have been used successfully for many years. These materials are known to improve the cutting tool performance of tungsten carbide in metal machining applications. Such coatings are produced primarily by two methods: (1) chemical vapor deposition, and (2) physical vapor deposition or sputtering. Even though these methods produce very pure coating materials, they cannot produce usable quantities of powder. Only thin coatings on solid materials can be made. Moreover, the coating process is very slow and requires precise control over the reaction conditions. To the extent that powder is formed, it occurs only as a secondary reaction.
Methods to produce titanium nitride, carbide and carbonitride powders have basically used three general approaches: (i) reactions using plasma, (ii) reactions using TiO.sub.2 and similar materials, and (iii) reactions using titanium halide, such as TiCl.sub.4, and a nitrogen or carbon containing gas.
Plasma reactions are based on reducing stable titanium compounds with hydrogen in the presence of a reactive gas. The plasma processes usually produce very fine powders. Some processes react pure titanium powder in the plasma and rely on the increased rate of reaction at high temperature to form the nitride or carbide powder. However, due to the high temperatures used, special equipment is needed to contain the reaction. Plasma generation is very energy intensive and, therefore, expensive to operate. Moreover, these processes have limited feed rates, thus limiting production.
The oldest processes for making titanium carbide and nitride use particulate TiO.sub.2 or similar materials as starting reactants. These reactions depend on reducing TiO.sub.2 with carbon in a nitrogen atmosphere if TiN is the desired product or with excess carbon if TiC is the desired product. Such reactions are typically very slow, requiring up to 10 to 20 hours for completion. A coarse powder is produced which requires milling to obtain the proper particles size and distribution. Moreover, because oxygen is part of the starting material, it always contaminates the final product. For example, in U.S. Pat. No. 1,391,147, von Bichowsky et al disclose a process of synthesizing titanium nitride by forming ground titanium dioxide, carbon, an alkali metal salt and metallic iron into briquette form, drying in an oxygen free environment and heating at 1000.degree. C. in a stream of nitrogen gas. In U.S. Pat. No. 3,036,888, Lowe teaches a method for producing titanium nitride by admixing particulate titanium dioxide and titanium carbide and heating the finely divided mixture at temperatures above 1500.degree. C. while a stream of nitrogen is passed therethrough. In U.S. Pat. No. 2,819,152, Aagaard discloses a method for forming titanium carbide from a starting titanium sulfate solution by hydrolyzing the sulfate solution in the presence of finely divided carbon particles, treating the resulting hydrated titanium compound/carbon coalesced particulate mixture with an alkaline metal hydroxide and calcining the treated mixture.
Several processes have been proposed based on the reaction between TiCl.sub.4 and NH.sub.3 or N.sub.2. These reactions require very high temperatures to nucleate the powder in free space. Very poor conversions are obtained even at temperatures above 1,300.degree. C. If reactions are conducted at lower temperatures, nonstoichiometric compounds are formed. To date, there are no commercial processes using this approach. Exemplary of this type process is U.S. Pat. No. 2,606,815 -- Sowa in which titanium nitride is prepared by forming the addition compound of titanium tetrachloride and ammonia, adding aqueous sodium fluoride and sodium hydroxide thereto to render the solution alkaline and to form a gelatinous precipitate, filtering and drying the precipitate, and fusing the dry material at 750.degree.-850.degree. C. to yield titanium nitride. In another process disclosed in U.S. Pat. No. 3,615,271 - Dietz, titanium carbonitride powder is made by reacting liquid titanium tetrachloride and a stoichiometric excess of an ethylamine in an inert atmosphere at temperatures up to 136.degree. C., heating the reaction product to about 600.degree. C. in an atmosphere of ammonia or methylamine, increasing the temperature to about 900.degree. C. in an atmosphere of hydrogen alone or combined with argon, methane or ammonia, and increasing the temperature to 1200.degree. C. in an argon atmosphere. According to still another process for preparing titanium carbonitride in powder form disclosed in U.S. Pat. No. 4,247,529 - Mori et al, a powdered mixture of titanium di- or tri-halide and carbon is heated to 700.degree. to 1800.degree. C. in the presence of a powdered aluminum or aluminum-titanium reducing agent in a nitrogen, nitrogen-hydrogen or ammonia gaseous atmosphere.
A recent process disclosed by Holt in U.S. Pat. No. 4,446,242, which produces titanium nitride or carbonitride uses metallic titanium as a starting material. In this process, Ti is mixed with NaN.sub.3 and ignited in a N.sub.2 atmosphere. However, in order to produce a fine-titanium nitride, a fine-titanium powder is necessary. Oxygen contamination is inherent in fine-titanium powder and, therefore, carries over into the TiN. Also detracting from the desirability of the process if the fact that sodium azide is an expensive material which is difficult to produce in pure form.
Another process, taught in U.S. Pat. No. 2,672,400 - Jacobson, forms titanium nitride by reducing TiCl.sub.4 with sodium to give a molten mixture of TiCl.sub.2 and NaCl and adding ammonia to the molten mixture to complete the reduction and nitride the resulting titanium at temperatures in the range 600.degree. to 850.degree. C. Thermodynamically, H.sub.2 cannot reduce TiCl.sub.2 at temperatures between 50020 and 1000.degree. C. This makes the last step in the process very slow and obtaining a homogeneous product that is completely reacted is difficult. Moreover, this process, as described, is a batch reaction with multiple steps which increase its complexity and cost.
It is, therefore, apparent that there exists a need in the art for a rapid, efficient and economical process for producing very finely divided titanium nitride, carbide and carbonitride powders of excellent purity and composition. Accordingly, it is the purpose of the present invention to provide such a process which is very rapid, going to completion within seconds, easily automated, readily operated on a continuous basis, and affords control over the composition of mixed carbon-nitrogen compounds. The process of the present invention produces homogeneous, fine grain low-oxygen, stoichiometric composition powders of titanium carbide, nitride and carbonitride which can be used to make hard abrasion resistant materials for sintered tools or for blending with other alloying elements for making ceramic components.